It is known in the art relating to automotive engines, that a key gasoline characteristic for good driveability is volatility. Volatility is especially important at the time an engine is started because liquid gasoline must evaporate and mix with air to form a combustible mixture. If too little gasoline is added, the engine will not start; if gasoline beyond that needed to initiate combustion is added, then extra hydrocarbons from an unburned portion of gasoline are found in the exhaust. Moreover, because gasoline sold in the United States varies in volatility, there is a tradeoff in engine design between low hydrocarbon emissions and good driveability with low volatility fuel.
To describe the effect of gasoline volatility on the cold start and warmup driveability of a vehicle, a driveability index (DI) has been developed. DI is a measure of volatility that was developed to help predict how different volatility gasolines affect engine performance and is correlated to how often an engine might stall, stumble, surge or hesitate. Other measures of volatility are also used. For gasoline that does not contain oxygenates, such as ethanol or methyl tertiary-butyl ether (MTBE), the definition of DI is based on a laboratory test (American Society for Testing and Materials D 86) in which a sample of gasoline is distilled as the temperature of the sample is raised. The fraction distilled is measured as a function of temperature andDI=1.5T10+3T50+T90  (1)where Tx is the temperature in degrees Fahrenheit at which x % of the gasoline sample has been distilled.
Since DI was developed for use with non-oxygenated fuels, a modified equation is needed to account for the addition of oxygenates into the fuel. For example, with 10% ethanol (by volume) added to oxygenate-free gasoline, an expression that provides better correlation to engine performance is:DI=1.5T10+3T50+T90+86  (2)
Another popular oxygenated compound additive in fuel is MTBE (methyl tertiary butyl ether). To account for the addition of 15% MTBE (by volume) in oxygenate-free gasoline, a constant of 43.2 is added to the right-hand side of Equation (1).
With the knowledge of the fuel driveability index value, proper amounts of fuel can be dispensed to the engine, which can produce smoother engine starts and reduce emissions from the engine during the start up cycle. However, it is not particularly practical to use the actual ASTM D 86 apparatus to perform a DI measurement on-board a vehicle.